1. Field of the Invention
The present invention relates to communication networks and, more particularly, to a method and apparatus for simplifying the computation of alternate network paths in a routed network.
2. Description of the Related Art
Data communication networks may include various computers, servers, nodes, routers, switches, bridges, hubs, proxies, and other network devices coupled to and configured to pass data to one another. These devices will be referred to herein as “network elements.” Data is communicated through the data communication network by passing protocol data units, such as Internet Protocol packets, Ethernet Frames, data cells, segments, or other logical associations of bits/bytes of data, between the network elements by utilizing one or more communication links between the devices. A particular protocol data unit may be handled by multiple network elements and cross multiple communication links as it travels between its source and its destination over the network.
The various network elements on the communication network communicate with each other using predefined sets of rules, referred to herein as protocols. Different protocols are used to govern different aspects of the communication, such as how signals should be formed for transmission between network elements, various aspects of what the protocol data units should look like, how protocol data units should be handled or routed through the network by the network elements, and how information such as routing information should be exchanged between the network elements. Protocol Data Units will also be referred to herein as packets.
There are several different types of network routing protocols, one class of which is commonly referred to as link state routing protocols. Link state routing protocols assign cost metrics to each link on the network, and the routers advertise the links and costs through the use of link state advertisements. The routers collect the link state advertisements and build a link state database containing information associated with links on the network. This network view enables the routers to compute lowest cost paths through the network to intended destinations. These calculations are performed in advance and then programmed into the data plane of the network element. In operation, when a packet arrives, the data plane will automatically forward the packet over the lowest cost path toward its intended destination. Several examples of routing protocols that operate in this manner include Intermediate System to Intermediate System (IS-IS) and Open Shortest Path First (OSPF), although other link state routing protocols exist and may be developed as well.
Network failures, such as link failures and node failures, may occur in a communication network. When a failure occurs, traffic that is intended to flow through the failure must be routed around the failure so that it is able to traverse the network. Many different ways of handling network failures have been devised over the years. For particular types of traffic, it is generally considered desirable or necessary to enable traffic to be switched to an alternate path with 50 milliseconds of failure on the primary path, so that real time traffic being carried by the network is not affected by the failure. While this is generally possible using physical layer protection switching, such as by switching traffic to a protection path using SONET equipment, it would be advantageous to be able to provide this type of protection at the routing layer.
When a failure occurs in a network implementing a link state routing protocol, the local router will react to the failure by generating and flooding new routing updates to other routers in the network, perhaps after a hold-down delay. Upon receipt of the routing update, all the routers in the network will re-compute routes through the network based on the new network topology. These routers will then load the revised forwarding tables into the forwarding hardware. The convergence time for this process to complete may last for several seconds. Accordingly, use of the link state routing protocol to provide sub-50 ms failure recovery is generally not tenable.
One relatively recent way to provide fast failover to an alternate path is through the use of pre-computed alternate paths. For example, when the router initially computes a path to a destination, the router may also assume a failure on its primary path and compute an alternate path to the destination at the same time. The alternate path may then be programmed into the data plane of the network element so that, if a failure on the primary path occurs, the alternate path may be used to forward traffic temporarily while new primary paths are being computed.
FIG. 1 shows a simplified example of a network 10 including six nodes 12 interconnected by links 14. The cost of the links in this figure will be assumed to be symmetric, and are shown as numbers on the links. In an actual implementation, the network may be much larger and include a larger number of nodes. In this example, traffic is flowing from R1 to R6. Initially, a shortest path 20 from R1 to R6 will be through nodes R2 and R3, since this path has a cost of 4.
FIG. 2 shows the network of FIG. 1 in which there has been a failure on the link between R2 and R3. Although R2 will sense the failure, R1 will not know of the failure and will continue to send traffic to R2 to be forwarded to R6. To enable R2 to continue to forward traffic to R6, R2 will have pre-computed an alternate path through the network and have programmed that alternate path into its data plane. For example, in the example shown in FIG. 2, the pre-computed alternate path may be to transmit the data to R4, which may then transmit the data to the destination (R6) over its own shortest path.
The failure on link 2 will eventually be advertised by R2 using a standard Link State Advertisement (LSA), so that each of the nodes on the network may recompute paths through the network using the updated network information. These new paths will then be used by the network elements in a standard manner. For example, since the path from R1 to R6 via R4 and R5 has a cost of 6, R1 will stop sending R2 traffic intended for R6 once the new paths are computed and installed by the network elements. Having pre-computed alternate paths, however, enables the network elements to continue forwarding traffic to intended destinations while the network nodes recomputed new primary paths through the new network topography.
To determine which paths are able to be used to forward traffic in this manner, each router must determine which neighbors are loop free. In the networks shown in FIGS. 1 and 2, R4 is loop free because the distance from R4 to R6 is less than the distance from R4 to R2 plus the distance from R2 to R6. Stated another way, when R2 sends traffic to R4 to be forwarded to R6, R2 needs to know that the shortest path from R4 to R6 does not require R4 to forward traffic back through R2. Since R4 will not know of the failure on the link from R2 to R3, having R4 return the traffic to R2 would cause a routing loop to occur, and would defeat the purpose of trying to send out the traffic to R6 via an alternate path through the network. Accordingly, to enable alternate path routing to be enabled, each router must determine which of its neighbors has a lowest cost path to a particular destination that does not cause traffic to be forwarded back through it, i.e., the nodes must determine which neighboring routers are on loop free paths to the intended destination.
Additionally, the nodes may preferentially select from available loop free neighbors for example by determining which neighbor's shortest path to the destination avoids the immediate downstream node R3. Selection of alternate paths is described in greater detail in U.S. Patent Application Publication No. US2005/0073958A1, the content of which is hereby incorporated herein by reference.
FIG. 3 shows an example where it will be assumed that R2 does not have any loop-free neighbors. In this case, R2 may be able to use router R1 as its alternate path if router R1 is configured to enable U-turns to occur in the event of a failure on the network. Enabling U-turns of this nature is also described in greater detail in U.S. Patent Application Publication No. US2005/0073958A1, the content of which is hereby incorporated herein by reference. Essentially, U-turns enable the number of source/destination pairs that are protected on the network to be increased. To enable U-turns, R1 must be capable of breaking U-turns, and must have a loop free node-protecting alternate path to reach the destination.
To enable a node to determine which neighbor should be used as the first hop in an alternate path to a particular destination, the node will run an algorithm such as the well-known Dijkstra algorithm rooted at the neighbor, to enable it to determine the shortest path from the neighboring node to the destination. This must be done for each neighbor. As the number of neighboring nodes increases, the processing requirements to compute the alternate paths increase significantly. Accordingly, it would be advantageous to provide a way to simplify the manner in which a reliable alternate path could be determined from a given node to an intended destination.